Personally I use a Swedish steel fillet knife as recommended by a knowledgeable skipper a few years ago. It's generally good for quite a few jobs before it needs refreshing. I do this with a decent sharpening steel; you can get them for £8 - 10 but if you want one with decent hardness you have to pay a bit more, worth doing because they last for years. Mine is a "Richardsons" and if you search around on the net you should be able to get a shorter one rather than the traditional 30 cm jobs.